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Aug. 7, 2012 -- People who are overweight or obese when they are diagnosed with type 2 diabetes appear to live longer than people whose body weight is normal when their disease is detected, a new study shows.

Obesity increases the risks for illness and early death. Despite this, doctors have long puzzled over why bigger patients with certain chronic diseases seem to fare better than those who are thin. This so-called "obesity paradox" has been noted in patients with kidney disease, heart failure, and high blood pressure.

The new study, which is published in the Journal of the American Medical Association, suggests the protective effect of a higher body mass index (BMI) may also extend to people with type 2 diabetes. BMI is a measure of size that accounts for both height and weight.

"This was unexpected given the close association of diabetes with obesity," says researcher Mercedes R. Carnethon, PhD, an associate professor of preventive medicine at Northwestern University's Feinberg School of Medicine in Chicago.

Excess body fat worsens the body's ability to use insulin properly, which affects blood sugar control. People with diabetes who are overweight are routinely advised to lose weight to help keep their disease in check.

Carnethon cautions that this study doesn't mean that people with diabetes who are overweight should abandon their weight loss efforts.

Instead, experts say the study suggests that people who are normal weight when they are diagnosed may be at increased risk of poor health outcomes, though doctors don't fully understand why.

"If you are normal weight, you may be at higher risk from diabetes, especially if your fitness status is not so good," says Hermes Florez, MD, PhD. Florez is the director of the division of epidemiology and population health sciences at the University of Miami's Miller School of Medicine. He wrote an editorial on the study but was not involved in the research.

"It's not just the issue of fatness. It's also the issue of fitness," he says.

Body Size and Death Risk

For the study, researchers pooled data from five different studies of heart disease. During the course of those studies, 2,600 adults over age 40 were diagnosed with diabetes. A total of 293 people (11.2%) had normal weight based on body mass indexes (BMIs) at the time of their diagnosis.

Even after accounting for health risks, like smoking, high bad cholesterol, waist size, and high blood pressure, people who had normal BMIs were about twice as likely to die during the studies compared to people who were overweight or obese.

The study wasn't able to tease out what it was about normal-weight people with diabetes that might have made them less healthy than those who were overweight or obese, but researchers have some theories.

Body Composition, Fat Distribution May Trump Body Size in Diabetes

One is body composition -- the ratio of fat to muscle. Muscle is critical to controlling blood sugar because it is metabolically active, uses insulin, and burns sugars and calories.

"The muscle-versus-fat ratio is extremely important for diabetes development as well as health outcomes related to diabetes," Carnethon says.

Studies show that it's becoming more common for normal-weight people to carry less muscle and more body fat.

Doctors have even coined a term for this: TOFI, or thin outside, fat inside. It's especially common in older adults who naturally lose muscle and bone with age.

"It could well be that these people do have an adverse body fat distribution. They haven't measured it in this study, so you can't be 100% sure, but it would fit into the general idea that these people have an adverse fat distribution. There could be more on the inside," says E. Louise Thomas, PhD, a research scientist at University College London. Thomas studies body fat and metabolism, but she was not involved in the research.

"What may be very significant is not just the actual weight, but what's in that weight. What's the ratio between muscle and fat and where is that fat stored?" says Rifka C. Schulman, MD, an endocrinologist at Long Island Jewish Medical Center in New Hyde Park, N.Y.

Normal-weight people may also get a different kind of diabetes than people who are overweight. Whatever the reason, experts say the study should be a wake-up call to clinicians that normal-weight people with diabetes need close attention.

"I think normal-weight people get overlooked to a certain extent because traditionally, that hasn't been where the problem is," Thomas says.